Electronic ballasts are used in many fluorescent lighting applications. Electronic ballast circuits typically employ transistors or other semiconductor components to convert mains voltage into high-frequency AC while also regulating the current flow in the lamp. The design and operation of these circuits include programming a plurality of parameters relating to the control of the fluorescent lamp, including lamp soft-start frequency, a pre-heat frequency, ignition frequency and ramp time, and final running frequency. These various frequencies are often generated by the ballast control circuit using a voltage controlled oscillator (VCO) which provides different frequencies for an output operating signal based on a voltage level or current provided at the input of the VCO. The power to the lamps is controlled by varying the output frequency of the VCO. Another timing circuit may be provided to program the time period during which the operating signal remains at each of the frequencies above, and yet another circuit is often provided to program the sweep time between each frequency.
Programming for rapid-starting often involves preheating a fluorescent lamp's electrodes using a higher pre-heat frequency before the required voltage for striking the arc is applied at the lower resonance frequency. The lamp will light after a predetermined preheat time as the frequency sweeps through the resonance frequency during the ignition ramp time. This can significantly increase the expected life of the lamp compared to instant-starting a lamp by applying the jolt of required voltage without a warm-up period, which may increase erosion of the electrode and result in fewer lamp starts before failure. Modern lamps require an accurate preheat time for a more controlled start. If the minimum frequency has been chosen below or very close to the resonant frequency, the circuit will work near resonance.
Electronic ballasts may employ two power MOSFETs driven to conduct alternately in a totem pole (half-bridge) topology in conjunction with L-C series resonant circuits, with the fluorescent lamp(s) across one of the reactances. Many electronic ballasts for fluorescent lighting applications sold on the market today also include a standard ballast control IC. There are a number of electronic ballast control ICs on the market, including the IR215X and IR2520D series which are monolithic power integrated circuits capable of driving low-side and high-side MOSFETs or IGBTs from logic level, ground referenced inputs, and which can provide self-oscillating or synchronized oscillation functions set via external resistive and/or capacitance components.
At startup, the output frequency of the VCO can be as much as about 2.5 times the minimum frequency of the circuit. This minimizes voltage spikes and lamp flash at startup. The frequency ramps down towards the resonant frequency of the high-Q ballast output stage, causing the lamp voltage and lamp current to increase. During this time, the filaments of the fluorescent lamp are pre-heated to the emission temperature to promote a long lamp life. The frequency keeps decreasing until the lamp ignites. If the lamp ignites successfully, the ballast control IC enters a run mode. If the minimum frequency has been chosen below or very close to the resonant frequency, the integrated circuit will work near resonance and will adjust the frequency to maintain zero-voltage switching at the half-bridge and to minimize the losses in the transistors. If the minimum frequency has been chosen higher than the resonant frequency the ballast control IC will work at the minimum frequency.
Control ICs can perform a variety of functions ranging from power factor correction to lamp/ballast control. The parameters of soft-start frequency, pre-heat frequency, ignition ramp time, and final run frequency may be programmed independent of each other using multi-input solutions.